For a website that supports multiple languages, it is important to tell the client
what the selected language is. This is done in the <html> tag in the lang or
xml:lang attribute and could look something like this for an XHTML document:

The DOCTYPE and <html> tag is usually static text that doesn’t get generated
by the server, but for localized websites we need to adjust the lang or xml:lang attribute
to make it correspond to the selected language.

In ASP.NET this is very easy because the thread knows about the selected language,
so all we have to do is to print it out in the attribute in the <html> tag.
You can do it easily by inserting directly on the aspx page like so: